This invention relates to signal filtering and, more particularly, to signal filtering of discrete-time signals. For a general discussion of discrete-time (sampled) filters and particularly of discrete-time and amplitude (digital) filters and some of their application, reference is made to Digital Processing of Signals by B. Gold and C. M. Rader, McGraw-Hill Book Company, 1969.
Recursive filters comprise a powerful class of filters because they can be easily designed to have sharp cutoffs and because high order recursive filters can be constructed by concatenating low order filters. Nonrecursive filters, on the other hand, can be designed to have a linear phase response but cannot obtain sharp cutoffs without the use of filter orders which are much higher than the filter orders of equivalent recursive filters. Because of the increased hardware required for sharp cutoff filters, nonrecursive filters have not, heretofore, been extensively used in the filter art. With the advent of large scale integrated circuit technology, however, this impediment of nonrecursive filters is circumvented because the circuitry can be integrated and thus be made conveniently small and economical. The one problem which still remains with high order nonrecursive (transversal) filters is the large number of leads that need to be provided between the integrated shift register stages required in the filters and the arithmetic circuits of the filters. Leads occupy space, require pins for connecting to the integrated circuits, and require internal interconnections. This translates into increased costs.